Although many cells and neural circuits clearly contribute to opiate and other substance abuse disorder, the path to drug addiction travels through midbrain dopaminergic neurons. Though a rare cell type (it is estimated that a mere 1 of every 200,000 neurons in the human brain is of a dopaminergic phenotype), changes in dopaminergic neurotransmission are thought to play a role in various stages of addiction, from acute reward mechanisms and goal-directed actions, to the development of habitual behavior and increased salience of cues associated with drug use, as well as the anhedonia and dysphoria associated with drug withdrawal. Surprisingly little is actually known about persistent changes in gene expression that presumably underlie the dysfunction of dopamine systems in brain exposed to opiates and other drug of abuse. Our project is centered on three Specific Aims. In Aim #1,we will extract chromatin from immunotagged midbrain dopaminergic neuron nuclei collected by fluorescence-activated sorting from 150 controls and 150 cases diagnosed with opiate abuse and then profile, on a genome-wide scale, the transcriptome and open chromatin landscapes and promoter-enhancer loopings and other types of chromosomal conformations (the ?3D genome?) in cell type-specific manner. In Aim #2, we will apply integrative genomics approaches and leverage Aim #1 postmortem brain data with population-scale genotypes and phenotypes provided by the Million Veterans Project and the Psychiatric Genomics Consortium to build causal probabilistic networks and predict key drivers within the regulatory non-coding DNA space of the dopaminergic system. In Aim #3, we will validate addiction-relevant cis-regulatory sequences (from Aim #1, #2) with small RNA-guided epigenomic editing systems in cultured human dopaminergic neurons. Collectively, our midbrain dopaminergic neuron- focused project will fill critical voids in the field of human addiction research and human neurogenomics and embark, for the first time, on a deep epigenomic assessment of one of the key cell populations in reward and addiction circuitry.